Fixed toFormData regression bug (unreleased) with Array-like objects serialization; (#4714)

Added `toURLEncodedForm` helper;
Added automatic payload serialization to `application/x-www-form-urlencoded` to have parity with `multipart/form-data`;
Added test of handling `application/x-www-form-urlencoded` body by express.js;
Updated README.md;
Added missed param in JSDoc;
Fixed hrefs in README.md;

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/helpers/toURLEncodedForm.js

@@ -0,0 +1,18 @@
+'use strict';
+
+var utils = require('../utils');
+var toFormData = require('./toFormData');
+var platform = require('../platform/');
+
+module.exports = function toURLEncodedForm(data) {
+  return toFormData(data, new platform.classes.URLSearchParams(), {
+    visitor: function(value, key, path, helpers) {
+      if (platform.isNode && utils.isBuffer(value)) {
+        this.append(key, value.toString('base64'));
+        return false;
+      }
+
+      return helpers.defaultVisitor.apply(this, arguments);
+    }
+  });
+};